EE 501 Lab7 Bandgap Reference Circuit

Transcription

1 Objective: EE 501 Lab7 Bandgap Reference Circuit 1. Understand the bandgap reference circuit principle. 2. Investigate how to build bandgap reference circuit. Tasks and Procedures: The bandgap reference voltage generator is designed to provide a stable reference voltage across the device operating temperature and voltage. The design uses a known BGR circuit, but replaces the PN junction diodes with MOSFET connection diodes. The proposed bandgap voltage reference consists three circuit blocks: a opamp, a bandgap core, and a startup circuit. The Opamp you designed in lab 4 could be used in this lab, which is sufficient for bandgap reference circuit. The bandgap reference circuit is used to generate a temperature independent voltage and is shown below. A key element in a bandgap circuit core is the diode. Unfortunately, AMI 0.5um technology does not have diode models in its library. In this work a PMOS diode connected elements have been used. The drain, gate, and source of the PMOS tied together to form the Anode and the body of the PMOS forms the Cathode. Figure 7.1 shows the PMOS diode used in proposed design. Figure 7.1 Forming a diode irk a PMOS transistor. The PMOS diode has the characteristic below: 1. Exponential relationship of current to voltage across holds. 2. PMOS diode exhibits a negative temperature coefficient similar to regular diodes.

2 3. The number of parallel component s N affects the cut-in voltage. The bandgap core provides the negative and positive temperature coefficients thru the diode elements. I1 I2 I1b Va Vb I2b I1a dvf I2a Vf1 Vf2 N Multiplier I3 Figure 7.2 Bandgap reference circuit The concept of the proposed BGR is that two currents, which are proportional to and, are generated by only one feedback loop. Fig7.2 presents the BGR circuit. The NMOS transistor dimensions are the same. The op-amp is so controlled that the voltages of and are equalized Therefore, the gates NMOS are connected to a common node so that the current, and becomes the same value due to the current mirror = = In this case, = and =

3 is proportional to is proportional to Here, is the sum of and, and is mirrored to Therefore, the output voltage of the proposed BGR, becomes is determined by the resistance ratio of, and and little influenced by the absolute value of the resistance. The NMOS are required to operate in the saturation region. Build the bandgap reference circuit in above and determine the resistance ratio to generate a bandgap reference voltage which is temperature invariant. You should also ensure that your op-amp is capable of driving the resistive loads. Here is a sample vs. Temperature plot. The is around 1.2V at room temperature. Figure 7.3 vs. Temperature

4 The temperature coefficient(tc) was calculated using equation ( ) ( ) ( ) Then sweep V DD from 2 to 3.3V and observe the independence with V DD. Figure 7.3 vs. V DD In simulation, you circuit may be able to start up easily. However, it doesn t guarantee that your circuit can start up after fabrication. So you should consider startup as a potential problem during the preliminary design phase. Build startup circuit as below or you can use other startup structure.

5 Figure 7.4 Bandgap reference circuit with startup circuit